This invention relates to article molding, particularly wherein plasticated material is injected into a die cavity and allowed to cure to a desired degree before being removed.
Injection molding techniques for forming plastic and rubber articles are well known. One such technique utilizes a clamping unit having a stationary die section and a movable die section. Opposing faces of these die sections are configured to define a die cavity when the sections are in abutting relationship. The stationary die section is provided with a sprue opening which communicates the interior of the die cavity with an injector unit capable of injecting a charge of plasticated substance into the die cavity.
A suitable type of injector unit may be of the reciprocal screw type. In such a unit, a screw is rotated within a barrel to melt and mix a predetermined charge of material, such as plastic or rubber for example. Subsequently, the screw is reciprocated, in the manner of a piston, to ram or "shoot" this charge through the sprue and into the die cavity.
Extremely high pressures are produced by such an injection operation. To resist these pressures, a heavy-duty clamping unit is provided to urge the die sections into tight interengagement. In many instances, the amount of counterforce or "clamp tonnage" necessary to maintain the die sections closed may be on the order of at least 20,000 psi, thus requiring the use of clamps of substantial size.
After being injected into the die cavity, the plasticated charge solidifies and may be subjected to a curing stage. Following this, the die sections are separated, and the molded article is suitably ejected.
While such apparatus is capable of producing articles of high quality, the quantity of articles which may be produced thereby is somewhat restricted, due mainly to the inactivity of the apparatus during the curing stage. The extent of this inactivity varies somewhat, depending upon the type of articles being molded. For example, in the production of large automobile body or bumper components, from synthetic rubber material, the period of inactivity may last for six minutes or more, during which time only a single article is being produced.
An effort to increase production, as by mounting a plurality of die pair on the platens to be injected simultaneously and cured simultaneously, would require the use of a larger injector in order to produce a sufficiently large charge for injecting two die cavities simultaneously. Moreover, the injection pressures required to be resisted by the clamp during this simultaneous injection would increase substantially, thereby requiring the use of much larger clamping apparatus.
It is, therefore, a general object of the invention to provide novel methods and apparatus for obviating or minimizing problems of the sort previously noted.
It is a particular object of the invention to provide such novel methods and apparatus which maximize the production rate of an injection-mold clamping unit while minimizing the required clamp tonnage.
It is another object of the invention to provide such novel methods and apparatus which maximize the production rate of an injection-mold clamping unit while minimizing the required clamp tonnage and injector capacity.
It is yet another object of the invention to provide such novel methods and apparatus which enable a charge to cure in the clamp unit simultaneously with the ejection of a cured charge therefrom.